1. Field of the Invention
This invention relates to relative humidity control instruments and in particular to an electronic humidity control system with capacitive humidity sensor.
2. Background Art
Heated holding cabinets for maintaining food in a heated or warm condition are commonly employed in restaurants, hotels, fast-food chains, and the like. Typically, the food is prepared or cooked in advance of its serving or sale and then stored in heated holding cabinets until needed. The holding cabinets are used to keep the food warm, and often moist, without cooking the food.
In some heated holding cabinets, means are not provided for varying the amount of moisture in the cabinet. It is desirable to control the amount of moisture in the heated, circulating air since the ideal moisture level varies from food to food. For example, where a crisp food is to be stored, less moisture or no moisture is required, whereas for moist foods greater amounts of moisture are required.
In the past, heated holding cabinets often employed water pans for adding moisture to the circulating air. These pans are filled at a sink, or the like, and then placed in the heated cabinet. Water is frequently spilled from these pans during transportation from water source to oven. With a water pan, the humidity of a cabinet can be controlled by adjusting the area of water exposed to heat. With the heated cabinet, a baffle member is employed within a water pan to substantially lessen the chance of water spillage; moreover, the baffle member provides control over the amount of moisture absorbed by the circulating air. It is difficult to control the cabinet humidity to an acceptable accuracy using this scheme.
Although these cabinets are equipped with a temperature control system, they usually are not equipped with an adequate humidity control system. Different food products require different storage conditions and accordingly different storage humidity. Thus cabinet humidity as well as cabinet temperature need to be controlled to provide adequate storage for food products.
Other prior art humidity control techniques include RH (relative humidity) sensors such as the one supplied by WATLOW CONTROLS in Winona, Minn. These sensors are only guaranteed up to 140.degree. F. and are fragile at temperatures above 140.degree. F. In the temperature range above 200.degree. F., these devices will be damaged and will not be able to operate to their specifications. This is a disadvantage because the operating range of most heated cabinets is from 140.degree. F. to 210.degree. F.
A capacitive humidity transducer which has an improved physical construction is disclosed by Suntola in U.S. Pat. No. 4,164,868. The primary problem with capacitive transducers prior to Suntola was that the dielectric film structure expanded and contracted during changes in water absorption and thus assumed a wavy rather than a planar shape. Thus, the dielectric film thickness could not be precisely determined. Suntola uses a base which carries at one of its surfaces a pair of electrically conductive coatings which are spaced from each other. The dielectric film is carried also by the surface of the base which carries the coatings, thereby making it possible to accurately determine the thickness of the dielectric film, and hence to accurately determine changes in the capacitance value of the capacitive humidity transducer.
Suntola does not disclose or suggest cooling one of the plates in the sensing capacitor. Further dew formation at high ambient temperatures is very difficult in the system of Suntola. Moreover, Suntola does not address measuring changes in capacitance due to ambient moisture. Suntola merely discloses new structure for a capacitive humidity transducer, without disclosing any means for measuring changes in capacitance.
U.S. Pat. No. 3,350,941 to Misevich et al. discloses a capacitance humidity sensing element. The element has a moisture insensitive, nonmetallic core having a high electrical resistivity and a low dielectric constant. Thin layers of coatings of a dielectric, highly moisture sensitive material are located on opposite surfaces of the core and are capable of absorbing water. The function of the moisture sensitive outer layers is to increase the sensitivity of the element, particularly at low humidity. The capacitance of the element is related to the partial water vapor pressure by virtue of the value of the dielectric constant of the moisture sensitive layers. The effective dielectric constant increases as the moisture content of the air increases, thus increasing the capacitance of the sensing element. The humidity sensing element is connected across a modified Wien bridge and the output of the bridge is connected through an amplifier to a volt meter which is calibrated to indicate the humidity.
Misevich does not disclose or suggest cooling one of the plates in the sensing capacitor. Accordingly, Misevich relies on absorption of moisture by the highly moisture sensitive material instead of a plate of the capacitor. Dew formation at high ambient temperatures is very difficult in the system of Misevich. Moreover, in Misevich, the change in capacitance is measured by a modified Wien bridge.
U.S. Pat. No. 5,272,963 to Del Fabbro discloses an arrangement for measuring moisture in ovens. Del Fabbro uses a temperature differential between two plates. Water vapor condenses on one of the two plates, causing the temperature differential. Del Fabbro does not disclose or suggest a sensing capacitor.
U.S. Pat. No. 5,365,039 to Chaudoir discloses a humidity controlled food warmer including a monitoring circuit. The monitoring circuit includes two capacitors. Chaudoir does not disclose or suggest a capacitor for sensing temperature based on formation of dews on a cool plate.
Thus, presently, a variety of apparatus and methods are employed to provide an adequate climate control for heated holding cabinets. However, these have not enjoyed wide spread acceptance, possibly due to their high costs and susceptibility at high temperatures. Accordingly, it would be appreciated to provide a humidity control system which is not only effective over a wider range of temperatures, but cost-effective and flexible enough to be installed in a variety of heated holding cabinets with different structural and climatic requirements.